Metallic foam structures (metallic solid foam or metallic cellular solids) are known to have interesting combinations of physical properties. Metallic foams offer high stiffness in combination with very low specific weight, high gas permeability, and a high energy absorption capability. As a result, these metallic foam materials are emerging as a new engineering material. Generally, foam structures can be classified as either open or closed porous. Open foams are mainly used as functional materials, such as for gas permeability membranes, while closed foams find application as structural materials, such as energy absorbers. However, the broad application of metallic foams has been hindered by the inability of manufacturers to produce uniform and consistent foam structures at low cost. Specifically, current manufacturing methods for producing metallic foams result in an undesirably wide distribution of cell and/or pore sizes which cannot be satisfactorily controlled. These manufacturing limits in turn degrade the functional and structural properties of the metallic foam materials.
The production of metallic foamed structures is generally carried out in the liquid state above the melting temperature of the material, though some solid-state methods have also been used. The foaming of ordinary metals is challenging because a foam is an inherently unstable structure. The reason for the imperfect properties of conventional metallic foams comes from the manufacturing process itself. For example, although a pure metal or metal alloy can be manufactured to have a large volume fraction (>50%) of gas bubbles, a desired bubble distribution cannot be readily sustained for practical times while these alloys are in their molten state. This limitation also results in difficulties in attempts to produce continuously cast parts with different thicknesses and dimensions.
Specifically, the time scales for the flotation of bubbles in a foam scales with the viscosity of the material. Most conventional alloys have a very low viscosity in the molten state. Accordingly, the mechanical properties of these foams are degraded with the degree of imperfection caused by the flotation and bursting of bubbles during manufacture. In addition, the low viscosity of commonly used liquid metals results in a short time scale for processing, which makes the processing of metallic foam a delicate process.
In order to remedy these shortcomings, several techniques have been attempted. For example, to reduce the sedimentation flotation process, Ca particles may be added to the liquid alloy. However, the addition of Ca itself degrades the metallic nature of the base metal as well as the resultant metallic foam. Alternatively, foaming experiments have been performed under reduced gravity, such as in space, to reduce the driving force for flotation, however, the cost for manufacturing metallic foams in space is prohibitive.
Accordingly, a need exists for improved methods of manufacturing amorphous metallic foams.